1. Field of the Invention
The present invention is directed to an energy efficient heat pump system capable of operating in environments with significant temperature variations.
2. Description of the Related Technology
Conventional heat pump water heaters (HPWH's) use about one-third to about one-half as much electricity as conventional gas and electric resistance water heaters, which only have an efficiency of about 60% to about 95% and a coefficient of performance of about 0.9. The electricity consumed by these HPWH's is used to transfer heat rather than to generate heat, thereby enabling high energy efficiency. Conventional HPWH's, however, have a number of limitations that prevent widespread adoption. Specifically, conventional HPWH's are only operable within a limited temperature range and are not suitable for use in cold temperature climates.
The schematic diagrams of FIGS. 1(a)-1(b) illustrate why conventional HPWH's are unable to provide a sufficient amount of heat in cold temperature climates to heat a hot water tank. Using an R-22 refrigerant, the condenser temperature of the HPWH shown in FIG. 1(a) during the spring, summer and fall seasons reaches about 21 bars, which is sufficient to produce hot water. However, as shown in FIG. 1(b), during the winter season, the evaporator pressure falls below 5 bars due to a decrease in the temperature differential between the temperature of the surrounding environment and the temperature of the refrigerant. Consequently, the condenser pressure drops to 19 bars with a saturation temperature of approximately 49° C. The maximum hot water temperature that may be achieved under this condition is 40° C., which is substantially below commercially acceptable water heater performance levels. Therefore conventional HPWH systems are recommended for use only in areas where the minimum surrounding temperature is above 7° C. Conventional HPWH's therefore typically require a back-up electric heater for directly heating a hot water tank when the surrounding environmental temperatures become too cold to enable effective heat transfer to the hot water tank from the HPWH.
To address this issue, some HPWH systems contemplate various methods for increasing the temperature within the HPWH system. For example, U.S. patent application publication no. 2009/0105889 discloses the use of a thermal expansion valve and supplemental heat exchangers for this purpose, and U.S. Pat. No. 7,159,419 discloses the use of a friction heat generator driven by an expander for this purpose. These systems, however, are unable to efficiently increase the temperature within the HPWH system and/or sufficiently increase the refrigerant pressure in the HPWH system.
In other HPWH systems, such as that disclosed in U.S. Pat. No. 7,543,456, an electric heater is used to heat the air flowing through an evaporator. In these systems, the electric heater is designed to remove frost that has accumulated on the evaporator coil rather than heat the refrigerant within the evaporator coil. An automatic control means turns off the heater when defrosting is complete.
Another limitation of conventional HPWH's is the fact that their energy efficiency decreases over time as scale and contaminants accumulate on the surfaces of the HPWH. Scale is particularly problematic in hard water regions and can reduce the overall heat transfer coefficient by 50% to about 80%. Although some HPWH systems claim to reduce scale build-up by swirling the water within a water tank, research has shown that scaling is not substantially solved or mitigated merely by such swirling.
Accordingly, there is a need to develop an improved heat pump system that is energy efficient, capable of operating in low temperature as well as high temperature environments and, optionally, capable of mitigating or preventing scale build-up. These and other objects and aspects of the invention will be apparent from the summary and detailed descriptions which follow.